The present invention relates to a light emitting diode comprising a light emitting diode element and a transparent cover covering the light emitting diode element partly or completely, and used for a display device, lighting, a backlight for liquid crystals, etc.
A light emitting diode (hereinafter referred to as the LED) is a light emitting device comprising an LED element that is a semiconductor light emitting element with a p-n junction, and has a small size and superior characteristics in terms of power consumption, light emission characteristic, durability, etc. Therefore, the LED is used for various indicators and light sources. At present, LEDs that emit light of various wavelengths, such as ultraviolet, blue, green, and red, have been developed. In particular, an LED comprising three LED elements for emitting red, green and blue lights respectively and capable of emitting any color by adjusting the emission intensity of each LED element has been put to practical use and is used in a full-color display, etc. Moreover, an LED that emits white light was developed by incorporating a plurality of LED elements having mutually different emission wavelengths into one LED, or combining an LED element with a phosphor for converting the wavelength of light emitted from the LED element, and such an LED is used for lighting, a backlight of liquid crystals, etc.
FIG. 1 is a front sectional view showing a conventional LED. The LED comprises lead frames 13 and 14, and the lead frame 13 has a recessed portion 13a in an end thereof. An LED element 11 constructed by stacking a plurality of semiconductor layers is adhered and fixed on the bottom of the recessed portion 13a by die-bonding. One of the electrodes of the LED element 11 is wire-bonded to the lead frame 13 by a metal wire 15, and the other electrode is wire-bonded to the lead frame 14 by a metal wire 15. The recessed portion 13a is filled with a transparent resin, so that a cover 12 covering the LED element 11 is formed. The end of the lead frame 13 where the cover 12 is formed and the end of the lead frame 14 are disposed in a mold 16 made of a transparent resin and having a top functioning as a convex lens. A transparent resin that forms the cover 12 contains a phosphor for converting the wavelength of light emitted from the LED element 11. For example, in the case where the LED element 11 is an LED element 11 that emits blue light and a phosphor for converting blue light into light of longer wavelengths is used, part of the blue light is converted into light of longer wavelengths and the LED emits white light. Examples of LEDs as described above are disclosed in Japanese Patent Publications Nos. 2927279, 3036465, and 3152238. In addition, a mono-color LED that does not have the cover 12, and an LED that does not have the cover 12 but has the mold 16 including a phosphor are widely used.
In a conventional LED, an epoxy resin is mainly used as a transparent resin for forming the mold 16, and an epoxy resin, silicone resin or the like is used as a transparent resin for forming the cover 12. Since resins such as an epoxy resin have a characteristic causing deterioration such as yellowing with light having a short wavelength, if an LED element 11 that emits light having a short wavelength such as blue light or ultraviolet light is used as the LED element 11, an LED comprising the cover 12 or mold 16 formed using such a resin has a problem that the light emission efficiency is lowered when used over a long period of time. Moreover, since resins such as an epoxy resin, silicone resin and acryl-based resin have insufficient moisture shield performance, an LED comprising the cover 12 or mold 16 formed using such a resin has a problem that the LED element 11 or the phosphor is deteriorated by humidity, and the color and luminous intensity change with use.
On the other hand, glass whose main component is silicon oxide is known as a transparent material which is superior to resins in terms of the resistance against deterioration caused by light of a short wavelength and the moisture shield performance. Since the melting point of soda glass used for common glass products is around 600° C., if the LED element 11 is covered with the melted soda glass and the soda glass is cooled and solidified to form the cover 12 or mold 16, the LED element 11 is damaged by the heat of the melted soda glass, and therefore the cover 12 or mold 16 can not be formed using soda glass. Further, even if lead glass containing lead, namely so-called low melting point glass is used, it is difficult to form the cover 12 or mold 16 without damaging the LED element 11 because the melting point of the lead glass is around 400° C. Japanese Patent Application Laid-Open No. 2001-48575 discloses low melting point glass having a melting point of 250° C. to 500° C. and containing no lead.